The present invention relates to improvements in a method of ultrasonic testing of objects such as pipes, strips, sheets, etc.
Ultrasonics is a well established technique for nondestructive testing of a variety of objects, particularly for inspecting the interior of the object for purposes of detecting any flaws and defects therein. One may distinguish here between two types of flaws or defects. One kind runs predominantly transversely to the surface of the test object, and one speaks here of transversal and of longitudinal flaws, depending upon the relation between the flaw and the predominant extension of the object. These flaws are usually detected through so-called transverse waves. The other type of defects or flaws extends predominantly parallel to a surface of the object and are usually detected by means of longitudinal waves.
The beam of ultrasonic transverse waves enters the test object usually at an angle between 33.degree. and 90.degree.. The most suitable angle used in each instance depends upon experience; different test objects and particularly differently contoured test objects have quite frequently their own and unique "best" entrance angle. For example, seamless pipes are tested with an entrance angle of the ultrasonic radiation beam in the range from 40.degree. to 55.degree., using particularly the single transducer pulse echo method. Testing of longitudinally or spirally seam welded pipes is carried out by means of test heads being oriented to produce entrance angles of 45.degree., 60.degree. and 70.degree.. The inspection of pipes requires beams which, from the interior of the pipe impinge upon the inner wall surface at 74.degree..
The relation of the most suited entrance angle does not just depend on the best suited orientation as far as the expected flaws is concerned, but one must consider the conditions under which such waves are produced and reflect at the test object. In particular, side effects of the beam may produce erroneous indications.
For example, transducers directing beams at particular angles towards the test object, produce also edge effect or fringe radiation which reaches the test object at different angles, and the conditions for reflection and propagation differ from those of the desired test beam. The reception of such waves by a pickup detector may simulate nonexisting defects.
If the angle of incidence is very steep, one produces increasingly longitudinal waves in addition to transverse waves, which may also lead to incorrect readings. If the angle of incidence is very shallow, e.g., more than 70.degree., edge effect radiation may produce surface waves which are very sensitive in surface near regions, and produce incorrect detection reading accordingly. All these side effects limit the choice of the entrance angle of a test beam into the test object, which, in turn, means that such angles have to be particularly set up and critically maintained.
As stated above, cylindrical objects such as rounds, rods, pipes, etc., are tested primarily by means of transverse waves. The particular entrance angle for the transverse waves can be obtained in various ways, and involved here is particularly the test head and transducer construction as well as the mode of coupling the transducer to the test object.
With very few exceptions, all methods, modes of coupling, and test head constructions, provide transverse waves into the object indirectly, i.e., by means of refraction of a longitudinal wave, being directly produced by the transducer. As the longitudinal wave impinges upon the surface of the test object, the wave is refracted and a transverse wave propagates into the test object. The desired entrance angle for that transverse wave is the refraction angle of the incident beam.
Cylindrical objects are tested by means of transverse waves resulting from obliquely positioned transducers which radiate at a fixed angle of incidence towards the test object. These angle beam heads have a particular coupler medium in which the ultrasonic wave fronts are produced at a particular angle in relation to a coupler surface. These heads produce one particular refraction angle for the entrance beam as propagating into the test object from the interface thereof with the coupler surface. This physical relation between transducer or test head and the test object is indeed a fixed one and results in a definite entrance angle for the test beam. However, this particular angle has validity only for a particular test material. The coupler medium and the orientation of the oscillator are fixed parameters. Thus, if such a head is applied to a different test object (different material), the angle may no longer be valid.
Automated test stands in a production line have their own particular requirements, including particular consideration for the life of the equipment. Also, these requirements involve, for example, changes such as a replacement or exchange of test heads for purposes of changing the needed entrance angles. Accordingly, a variety of systems have been developed which permit pivoting of the angle beam heads for purposes of changing the angles of incidence. These holders are often constructed in accordance with the type, contour, etc., of the object to be tested, and they are quite unique with the test object concerning the needed adjustment angles. Also, angle beam heads with variable entrance angles have been developed.
Another point to be considered is that often fluid coupler media are interposed between the test heads and the test objects. The test head if of angle type construction, includes usually a wedge element as stated to establish a particular relation between the direction into which the transducer oscillator transmits it vibrations, and the coupler surface from which the beam emerges. If a fluid is interposed, that coupler surface interfaces with the fluid and not with the test object. Hence, there are two interfaces for a beam, one between the transducer coupler element and the coupler fluid, and a second one between that fluid and the test object. The ultrasonic waves are refracted on both surfaces, but these two refractions, in effect, cancel each other out, so that again only the relationships between the solid media remains determining for the entrance angle of the beam into the test object. In other words, the entrance angle of the beam into the test object remains unaffected by the presence or absence of the coupler fluid. Still in other words, the coupler fluid does not operate as a modifier for the entrance angle of the inspection beam. Thus, the change of that angle is a matter of position adjustment of the transducer. It is quite detrimental here that the resulting entrance angle is quite difficult to reproduce and is also to be determined to begin with.